Dark Cloak still cancels the bonus speed from Concealed Weapon. Once the ability (dark cloak) wears off, you move slower than before you used it. You must leave sneak and re sneak in order to move at the correct speed. Sometimes trying to sneak again does not fix it and you must log out of game to reset it.

This has been the case for a while now.... it's a bug that messes up pvp and pve.